OPTI 202 L Lab 12 pn Junctions Photodiodes
OPTI 202 L Lab #12 – p-n Junctions: Photodiodes, Solar Cells LED’s, and Laser Diodes Dr. Mike Nofziger Professor College of Optical Sciences University of Arizona Dr. Mike Nofziger 2014
Lecture #12 Outline: ● Photodiodes—Physical Construction —Basic Properties —Basic Physics —Current Voltage Characteristics —Use in an Electrical Circuit ● Solar Cells—Basic Properties ● LED’s—Basic Properties Dr. Mike Nofziger 2014 Lecture 12
Photodiodes—Physical Construction: “Official Symbol” A 40 Gb/s “Optical Receiver” !! Dr. Mike Nofziger 2014 Lecture 12
Photodiodes—Basic Properties: ● p-n junction (p-side ≡ “anode”, n-side ≡ “cathode”) ● Built-in electric field (depletion region) separates the electrons and holes (electrons → p-side, holes → n-side) ● Photons absorbed (ideally in or near the depletion region) create electron-hole pairs ● Built-in electric field separates the electrons and holes before they recombine, producing a photocurrent (electrons → n-side, holes → p-side) ● I-V curve is very non-linear ● The photocurrent is linear with photon flux over 7 -decades! ● Most common semiconductor material used to make photodiodes (for detection of visible light) is Silicon (Si). Dr. Mike Nofziger 2014 Lecture 12
Photodiodes—Basic Physics: Dr. Mike Nofziger 2014 Lecture 12
Photodiodes—Current-Voltage Characteristics: The “Shockley diode equation” Io is the reverse saturation current V is the voltage across the junction Photocurrent generated by irradiance Ee (W/m 2) Photocurrent generated by optical power ϕe (W) Dr. Mike Nofziger 2014 Lecture 12
Photodiodes—Current-Voltage Characteristics: Dr. Mike Nofziger 2014 Lecture 12
Photodiodes—Use in an Electrical Circuit: Operated at V = 0 “zero-bias”: Output is very linear over 7 -decades of flux Operated at –V “reverse-bias”: Capacitance decreases, speed increases Operated at I≈0 “open-circuit”: The open-circuit voltage is logarithmic with flux: NOT the preferred way to operate a photodiode! Dr. Mike Nofziger 2014 Lecture 12
Basics of Solar Cells: ● A solar cell is a Photovoltaic (“PV”) detector: - is made of Silicon (not silicone!!) - absorbs light from ≈ 350 nm – 1100 nm - the absorption of light “frees up” electrons - This creates a voltage at the terminals of the cell (the “Open-Circuit” voltage) - If the cell’s terminals are shorted, the maximum current will flow (the “Short-Circuit” current) - If a load resistor is connected to the cell, a current will flow (the “Photocurrent”) Dr. Mike Nofziger 2014 Lecture 12
Basics of Solar Cells: ● The Power (Watts) that the cell can produce is given by: ● Because of internal resistance in the cell, the maximum power you can generate is across a load resistance equal to the internal resistance. www. keithley. com Dr. Mike Nofziger 2014 Lecture 12
“Sources of Light” LED – Light Emitting Diode – solid-state, semiconductor p-n junction – Ga. As, Al. Ga. As, Ga. P, Al. Ga. In. P, Ga. N, In. Ga. N, etc. – 3 mm, 5 mm, or 8 mm dia. plastic packages Low-Power LED’s 1 -20 m. A of electrical current “on/off indicators” High-power LED’s 500 m. A to >1 A Lumileds, Osram, Cree Lighting!! Replace incandescent bulbs Dr. Mike Nofziger 2014 Lecture 12
“Sources of Light” LED – Light Emitting Diode Dr. Mike Nofziger 2014 Lecture 12
“Sources of Light” White-Light LED’s: General Approaches Reference Dr. Mike Nofziger 2014 Lecture 12
“Sources of Light” White-Light LED’s (3 phosphors, “tri-phosphor”) – Al. Ga. In. N LED is the source of light – emits light in the near-UV spectral region – 380 -430 nm – 3 phosphor coatings on top – Europium-based red and blue phosphors – Copper and aluminum doped zinc sulfide green phosphor Reference – Color-mixing more easily controlled to produce a pure white light. – Example: CREE Dr. Mike Nofziger 2014 Lecture 12
“Sources of Light” White-Light LED’s (1 phosphor) – Ga. N semiconductor LED is the source of light – emits blue light 450 – 470 nm – Yellow phosphor coating on top – Cerium-doped YAG: Y 3 Al 5 O 12: Ce 3+ – light is emitted by scintillation (no afterglow) Example: CREE Dr. Mike Nofziger 2014 Lecture 12
“Sources of Light” White-Light LED’s: TRENDS Reference Dr. Mike Nofziger 2014 Lecture 12
“Sources of Light” White-Light LED’s: Applications vs. Dr. Mike Nofziger 2014 Lecture 12
“Sources of Light” White-Light LED’s: Applications Uses a CREE XM-L 1000 lumen LED Dr. Mike Nofziger 2014 Lecture 12
Laser Diodes—LI Curve Dr. Mike Nofziger 2014 Lecture 12
- Slides: 19